Hydrogen has many industrial uses that include the processing of sulfur containing crude oil within refineries to produce petroleum products. Hydrogen, itself, has more recently been considered as a possible substitute for petroleum fuels that are currently used in vehicles.
Hydrogen is commonly produced within hydrogen plants that have a steam methane reformer. Typically, natural gas is preheated and introduced into a hydrotreater containing a catalyst to reduce organic sulfur species into hydrogen sulfide and to a limited extent hydrogenate olefins into saturated hydrocarbons. The resulting treated feed is combined with superheated steam to produce a reactant stream fed to the steam methane reformer. As is well known in the art, the presence of sulfur containing compounds and a high olefin content within such a reactant stream will deactivate reforming catalyst within the steam methane reformer. Allowable levels of sulfur containing compounds and olefins within a reactant stream are commonly less than about 0.1 ppmv and less than 0.5 percent by volume, respectively, on a dry basis.
The reactant feed stream is then heated and fed to reformer tubes located within a furnace section of the steam methane reformer. Burners firing into the furnace section provide the heat necessary to support endothermic reforming reaction within the catalyst filled reformer tubes. The flue gas from the furnace section is then routed to a convective section of the reformer to preheat the reactant stream, to heat boiler feed water and to generate the superheated steam through indirect heat exchange occurring within heat exchangers located within the convective section. A reformed product discharged from the reforming tubes and containing hydrogen, carbon monoxide, steam, carbon dioxide, and methane is then cooled and processed within one or more water-gas shift reactors in which the steam is reacted with the carbon monoxide to increase the hydrogen concentration within the reformed product. The product hydrogen is then produced by cooling the hydrogen-rich stream from the water-gas shift reactor(s) and then separating the hydrogen in a pressure swing adsorption unit. The resulting tail gas is used, at least in part, as part of the fuel for the burners in the furnace section of the steam methane reformer.
A variety of off-gas streams are produced in refineries from processes such as fluidic catalytic cracking, coking, catalytic reforming and hydrocracking. These streams have a sufficiently high hydrocarbon and hydrogen content that they potentially could be reformed to produce a synthesis gas stream within the steam methane reformer. The problem with the use of such streams is that they have too high an organic sulfur species content and an olefin content to be directly utilized within a hydrogen plant by being passed through a conventional hydrotreater. While hydrogen plants have been designed with hydrotreaters capable of processing olefins, such hydrotreaters as a result can require recycle compressors and become much larger in volume and more difficult to operate in a reliable manner.
In order to overcome such limitations, U.S. Pat. No. 7,037,485 incorporates a reactor that utilizes a catalyst that is capable of promoting both hydrogenation and oxidation reactions. The reactor described in this patent is designed to process off-gas streams without a hydrotreater by hydrogenating the olefins into paraffins and reducing the organic sulfur species content to hydrogen sulfide. Alternatively, steam and oxygen can be introduced into reactor to produce saturated hydrocarbons, methane, carbon monoxide and hydrogen to increase the hydrogen output of the hydrogen plant. The limitation on the use of such a reactor is that it must operate at a sufficiently low space velocity to ensure that its product, either alone or after having been mixed with a desulfurized natural gas stream, contains no more than 0.1 ppmv of total sulfur (organic sulfur species and hydrogen sulfide) and less than 0.5 mol percent olefins, on a dry basis. In order to accommodate this limitation, the flow of the off-gas stream to be treated must be suitably limited or the reactor must be sufficiently sized to produce a sufficiently low concentration of total sulfur and olefins in the feed to the steam methane reformer.
As will be discussed, the present invention provides a method and apparatus that utilizes a sulfur tolerant catalyst, such as described above, that among other advantages, allows operations to be conducted at higher space velocities and therefore, permits the reactor to be more compact and less expensive and/or able to accommodate higher flow rates of hydrocarbon containing streams to be treated.